1. Technical Field of the Invention
The present invention relates to an illumination device including a light guide which receives light emitted by a light source at a light-receiving face of the light guide, guides the light to a light-emitting face of the light guide, and emits the light to the outside. The present invention also relates to a liquid crystal apparatus using the illumination device.
2. Description of the Related Art
Liquid crystal apparatuses are widely known as electro-optical displays. Generally, in the liquid crystal apparatus, liquid crystal is sandwiched between a pair of substrates each provided with electrodes, and the orientation of the liquid crystal is controlled by applying a voltage between the electrodes, thereby modulating light transmitted through the liquid crystal and displaying images.
When classifying the liquid crystal apparatuses according to the method of supplying light to the liquid crystal, various liquid crystal apparatuses are known, such as a reflective-type liquid crystal apparatus in which external light is reflected by a reflector provided on the outer or inner face of one of the pair of substrates, a transmission-type liquid crystal apparatus in which a planar light is applied to the liquid crystal by an illumination device provided on the outer face of one of the pair of substrates, and a semi-transmission reflective liquid crystal apparatus (i.e., transflective) which functions as a reflective-type liquid crystal apparatus when there is sufficient external light and as a transmission-type liquid crystal apparatus when not enough external light is applied.
The illumination device which is used in a transmission-type liquid crystal apparatus or semi-transmission reflective liquid crystal apparatus, as shown in FIG. 8, basically has a light source 71, such as an LED (light emitting diode) or a cold cathode tube, opposing a light-receiving face 74a of a light guide 74, introduces the light received by the light-receiving face 74a from the light source 71 into the light guide 74, guides the light while the light is reflected by a reflector 74b to a light-emitting face 74b, and emits the light to the outside from the light-emitting face 74b. A device, for example, a liquid crystal panel (not shown) which uses a planar light is disposed at the light-emitting face 74b, and the planar light is supplied to the device. Note that a reference symbol R shows conceptual light paths, and it does not show actual light paths.
Recently, color displays using liquid crystal apparatuses have become popular. In order to perform attractive display by using color displays, it is necessary that the light for illuminating the liquid crystal panel has high luminance. In particular, a luminance of app approximately 2 cd/m2 is required for a monochrome display. On the other hand, a high luminance of 10 cd/m2 or more is required for a color display. Moreover, a color display panel has a low light-transmissivity which is, for example, of the order of 2%. Therefore, the illumination device is required to generate high-luminance light.
Although, as described above, the illumination devices have been recently required to generate high-luminance emission light, the light introduced into the light guide 74 is inefficiently emitted to the outside from the light-emitting face 74b in the known basic illumination device shown in FIG. 8. Therefore, there has been a problem that emitted light having high luminance cannot be obtained.
A known illumination device is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 6-082631, in which unevenness in light generation in the vicinity of the edges of the illumination device is avoided with a diffusion member or a light-absorbing member disposed at an end face of a light guide. A known illumination device is also disclosed in, for example, Japanese Unexamined Patent Application Publication No. 8-320486, in which an end face of a light guide, facing a light source, is inclined, thereby increasing the light introduced into the light guide. Another illumination device is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 8-335048, in which a reflective sheet is mounted on a face opposite a light-receiving face of a light guide.
As described above, there are various proposals for avoiding unevenness in light generation and increasing light strength of an illumination device. However, there has been a problem in that it is difficult to generate a planar light having high luminance in the known illumination devices.
Accordingly, an object of the present invention is to provide an illumination device which can form a planar light having high luminance. Another object of the present invention is to provide a liquid crystal apparatus which can perform significantly clear and uniform displays.
To these ends, a first illumination device according to the present invention comprises a light source; and a light guide for receiving light from the light source at a light-receiving face of the light guide and emitting the light from a light-emitting face of the light guide, wherein a face opposite the light-receiving face of the light guide is formed as an inclined face. That is, the face opposite the light-receiving face of the light guide is angled relative to the light-receiving face.
In the thus formed illumination device, the light, which has been introduced into the light guide from the light-receiving face of the light guide and reaches the inclined face opposite the light-receiving face without being emitted to the outside from the light-emitting face of the light guide, reflects at an angle rather than reflecting directly. As a result, the number of times of reflection of the light, which is being transmitted inside the light guide, increases, thereby increasing frequency in diffusion, whereby the light-emission efficiency improves and the luminance of emitted light increases. Unevenness in the luminance of the emitted light is decreased by increasing the number of times of reflection of the light which is being transmitted inside the light guide.
The angle of inclination of the inclined face of the first illumination device is studied, as follows. The angle of an inclined face 6c is set to a value xcex8 with respect to a plane Pn (plane which extends vertical in the drawing) normal to a light-emitting face 6b of a light guide 6, as shown in FIG. 2(a), and light-emission efficiency was obtained from the following expressions by performing a simulation.
Light-emission efficiency (%)=(emitted-light amount/incident-light amount)xc3x97100,
in which emitted-light amount=amount of light emitted from light-emitting face 6b, and
incident-light amount=amount of light incident to light-receiving face 6a. 
The result is shown in FIG. 7(a). The angle xcex8 of inclination, in FIG. 2(a), is shown in a positive value when the inclination is in the clockwise direction and in a negative value when the inclination is in the counterclockwise direction.
A graph shown in FIG. 7(b) was obtained from the data of the result shown in FIG. 7(a). It is known from the graph that the light-emission efficiency is low when the angle xcex8 of inclination of an end face 6c opposing the light-incident side of the light guide 6 shown in FIG. 2(a) is 0xc2x0, as in a case of the known illumination device. As the angle of inclination of the end face increases, the light-emission efficiency gradually improves until the angle of inclination becomes approximately xc2x110xc2x0. However, the light-emission efficiency gradually decreases when the angle of inclination exceeds approximately xc2x110xc2x0.
That is, when the face 6c opposite the light-receiving face 6a of the light guide 6 is formed as an inclined plane, high light-emission-efficiency can be obtained when the angle xcex8 of inclination of the inclined face 6c is within a proper range. On the other hand, the light-emission efficiency cannot be significantly improved when the angle xcex8 of inclination is excessively large. The proper range of angle is between approximately +10xc2x0 and xe2x88x9210xc2x0.
Various reasons may be considered, why the light-emission efficiency cannot be improved in the light guide 6 when the angle xcex8 of inclination is excessively large. One reason may be that when the angle xcex8 of inclination is excessively large, the light reflected by the end face is immediately emitted to the outside of the light guide 6 from the vicinity of the end face; therefore, the number of times of reflection of the light in the light guide 6 cannot be increased. Accordingly, in the first illumination device, the angle of inclination of the inclined face 6c is preferably set to approximately xc2x110xc2x0 with respect to the plane Pn normal to the light-emitting face 6b of the light guide 6.
In the first illumination device, a reflective member is preferably provided on a face opposite the light-emitting face of the light guide. With this arrangement, the light incident on the light guide can be efficiently emitted to the outside from the light-emitting face. As a reflective member, for example, a white reflective sheet, which is formed independently of the light guide, may be bonded, a white reflective layer, for example, may be formed on a face of the light guide, or any other method may be applied.
In the first illumination device, a reflective member may be provided also on the inclined face opposite the light-incident face of the light guide. With this arrangement, the light incident on the light guide is prevented from leaking to the outside through the opposite inclined face, thereby further improving the emission efficiency. As a reflective member, for example, a white reflective sheet, which is formed independently of the light guide, may be bonded, a white reflective layer, for example, may be formed on a face of the light guide, or any other method may be applied.
(2) A second illumination device according to the present invention comprises: a light source; a light guide for receiving light from the light source at a light-receiving face of the light guide and emitting the light from a light-emitting face of the light guide; and diffusion patterns provided on a light-emitting face or a face opposite the light-emitting face of the light guide, wherein the face opposite the light-receiving face of the light guide is formed as an inclined face, and the pattern density of the diffusion patterns increases from the inclined face toward a middle part of the light guide. Thus, the face opposite the light-receiving face of the light guide is angled relative to the light-receiving face.
The pattern density is a ratio of an area, which is occupied by the diffusion patterns, per a unit area of the light guide. For example, in order to increase the pattern density, the size of the diffusion patterns may be increased, or density in pattern disposition may be increased without changing the size of patterns.
The second illumination device differs from the first illumination device in that the diffusion patterns are provided on the light-emitting face or the face opposite the light-emitting face, and the shape of the diffusion patterns is determined in connection with the inclined plane, in addition to forming the face opposite the light-receiving face of the light guide as an inclined face.
When a face opposite the light-receiving face, that is, the face opposite the light-incident face is formed as an inclined face, the light which reaches the inclined face reflects at an angle off the inclined plane. Therefore, the luminance of the light emitted to the outside from the light-emitting face of the light guide tends to increase toward the inclined plane, that is, the end face opposite the incident side of the light guide, whereby the uniformity of a planar light may be deteriorated. When the pattern density of the diffusion patterns is set so as to increase from the inclined plane, that is, the end face of the light guide toward a middle part of the light guide, as in the second illumination device, the luminance of the emitted light at the end face side of the light guide can be decreased, and the luminance of the emitted light at the middle part of the light guide can be increased, whereby the luminance of the emitted planar light can be made uniform.
With reference to FIG. 2(b) which is a schematic plan view of the illumination device, when the pattern density of diffusion patterns 12a disposed in the vicinity of a light source 7 is denoted by S0, the pattern density of diffusion patterns 12c disposed in the vicinity of the inclined face 6c opposite the incident side is denoted by S1, and the pattern density of diffusion patterns 12b disposed in a middle part of the light guide 6 is denoted by S2, the relationship between these pattern densities preferably satisfies an expression S0 less than S2 less than S2.
When the pattern density of the diffusion patterns is set, as described above, the light-emission efficiency in the vicinity of the LED 7 is lowest, the light-emission efficiency in the vicinity of the inclined end-face 6c which is considered as a dummy light-source is second lowest, and the light-emission efficiency at the middle part is maintained highest. As a result, the luminance of the emitted planar light from the light guide 6 is made uniform.
In the illumination device in which the diffusion patterns are formed on the light-emitting face or a face opposite the light-emitting face of the light guide in relation to the inclined end face, as described above, an expression L1 greater than L2 is preferably satisfied when L1 denotes a distance from the diffusion patterns 12a of which the pattern density is S0 and which are disposed closest to the LED 9 to the diffusion patterns 12b of which the pattern density is S2 and which are disposed in the middle part, and L2 denotes a distance from the diffusion patterns 12c of which density is S1 and which are disposed closest to the inclined end-face 6c to the diffusion pattern 12b of which the pattern density is S2 and which are disposed in the middle part, as shown in FIG. 2(b).
With this arrangement, the luminance at the end toward the light source side, which tends to be a highest luminance, is lowered, the luminance at the dummy light-source side, that is, the end toward the inclined face 6c side, which tends to be a second highest luminance, is moderately lowered, and the luminance at the middle part, which tends to be a lowest luminance, can be controlled so that the attenuation becomes as smallest as possible, whereby the luminance of the planar light from the light guide 6 can be made uniform.
The light source in each of the first and second illumination devices is preferably an LED (light emitting diode). Generally, a cold cathode-ray tube such as a fluorescent light or other light source such as an LED may be used. The LED among these light sources has a high directivity; therefore, a large portion of the component of the light generated by the LED and received by the light guide at the light-receiving face reaches the opposite end face without being emitted to the outside of the light guide, when the LED is used as a light source.
In this case, when any particular arrangement is not performed on the face opposite the light-receiving face, as in the known illumination device, the components of the light, which have reached the face opposite the light-receiving face, are reflected directly toward the light-receiving face and is not easily emitted to the outside of the light guide. Therefore, it is difficult to obtain a light having a high luminance emitted from the light guide.
When the face opposite the light-receiving face 6a is inclined, as in the present embodiment, the light which has reached the inclined plane can reflect at an angle, thereby reflecting multiple times inside the light guide, whereby emitted light having a high luminance can be obtained particularly from the LED which has high directivity.
(3) A first liquid crystal apparatus according to the present invention comprises: a liquid crystal panel comprising a pair of substrates sandwiching liquid crystal; and an illumination device for supplying light to the liquid crystal panel. The illumination device comprises a light source and a light guide which receives light from the light source at a light-receiving face of the light guide and emits the light from a light-emitting face of the light guide. A face opposite the light-receiving face of the light guide is formed as an inclined plane. That is, the face opposite the light-receiving face of the light guide is angled relative to the light-receiving face.
In an illumination device which is included in the liquid crystal apparatus according to the present invention, the light, which has been introduced into the light guide from the light-receiving face of the light guide and reached an inclined plane opposite the light-receiving face without being emitted to the outside from a light-emitting face of the light guide, reflects at an angle rather than reflecting directly. As a result, the number of times of reflection of the light which is being transmitted inside the light guide increases, thereby increasing frequency in diffusion, whereby the light-emission efficiency is improved and the luminance of emitted light is increased. Unevenness in the luminance of the emitted light is decreased by increasing the number of times of reflection of the light which is being transmitted inside the light guide. Since the luminance of the light from the illumination device can be increased and unevenness in the luminance can be decreased, clear images can be displayed uniformly in the overall region of a display.
(4) A second liquid crystal apparatus according to the present invention comprises: a liquid crystal panel comprising a pair of substrates sandwiching liquid crystal; and an illumination device for supplying light to the liquid crystal panel. The illumination device comprises a light source, a light guide which receives light from the light source at a light-receiving face of the light guide and emits the light from a light-emitting face of the light guide, and diffusion patterns provided on a light-emitting face or a face opposite the light-emitting face of the light guide. A face opposite the light-receiving face of the light guide is formed as an inclined plane, and the pattern density of the diffusion patterns increases from the inclined plane toward a middle part of the light guide. Thus, the face opposite the light-receiving face of the light guide is angled relative to the light-receiving face.
The second liquid crystal apparatus differs from the first liquid crystal apparatus in that the illumination device included in the second liquid crystal apparatus is modified. In particular, the diffusion patterns are provided on the light-emitting face or a face opposite the light-emitting face, and the shape of the diffusion patterns is determined in relation to the inclined plane, in addition to forming the face opposite the light-receiving face of the light guide as an inclined plane.
When the face opposite the light-receiving face, that is, the face opposite the light-incident face is formed as an inclined plane, the light which reaches the inclined face reflects at an angle off the inclined plane. Therefore, the luminance of the light emitted to the outside from the light-emitting face of the light guide tends to increase toward the inclined plane, that is, the end face opposite the incident side of the light guide, and therefore the evenness of a planar light may be deteriorated. When the pattern density of the diffusion patterns is set so as to increase from the inclined plane, that is, the end face of the light guide toward a middle part of the light guide, as in the illumination device used in the second liquid crystal apparatus, the luminance of the emitted light at the end face side of the light guide can be decreased, and the luminance of the emitted light at the middle part of the light guide can be increased, whereby the luminance of the emitted planar light can be made uniform. Therefore, clear images can be displayed uniformly in the overall region of a display of the liquid crystal apparatus.